Add test for drop order

tests/self_cell.rs

@@ -9,11 +9,13 @@ use std::panic::catch_unwind;
 use std::process::Command;
 use std::rc::Rc;
 use std::str;
+use std::sync::Mutex;
 
 use crossbeam_utils::thread;
 
 use impls::impls;
 
+use once_cell::sync::Lazy;
 use once_cell::unsync::OnceCell;
 
 use self_cell::self_cell;
@@ -692,3 +694,43 @@ fn try_build_manual(path: &str) {
 fn invalid_compile_manual() {
     try_build_manual("tests/invalid_manual/wrong_covariance");
 }
+
+#[test]
+fn drop_order() {
+    #[derive(Debug, PartialEq, Eq)]
+    enum Dropped {
+        Owner,
+        Dependent,
+    }
+
+    struct Owner;
+    struct Dependent<'a>(&'a Owner);
+
+    static DROPS: Lazy<Mutex<Vec<Dropped>>> = Lazy::new(<_>::default);
+
+    impl Drop for Owner {
+        fn drop(&mut self) {
+            DROPS.lock().unwrap().push(Dropped::Owner)
+        }
+    }
+    impl Drop for Dependent<'_> {
+        fn drop(&mut self) {
+            DROPS.lock().unwrap().push(Dropped::Dependent)
+        }
+    }
+
+    self_cell! { struct Foo {
+            owner: Owner,
+            #[covariant]
+            dependent: Dependent,
+        }
+    }
+
+    let foo = Foo::new(Owner, |o| Dependent(o));
+    DROPS.lock().unwrap().clear();
+    drop(foo);
+    assert_eq!(
+        &DROPS.lock().unwrap()[..],
+        &[Dropped::Dependent, Dropped::Owner]
+    );
+}